function plot_from_file(datafile)
%
% Make a simple x-y plot based on values read from a datafile.
% We assume that the datafile has four columns of floating-point
% values.  We plot column 4 versus column 2.
%   
% Arguments:
%              datafile        (input)   name of file containing data 
%                                           
% MWR 4/11/2001

% set debug = 1 to see lots of diagnostics
debug = 1;

% open the datafile (quit if we can't)
fid = fopen(datafile, 'r');
if (fid == -1) 
  error('cannot open datafile');
end

% loop over all lines in the file, reading them one at a time
num_lines = 0;
done_yet = 0;
while (done_yet == 0)

  % try to read the next line ...
  line = fgets(fid);
  if (line == -1)
    break;
  end
  [data_array, num_read] = sscanf(line, '%f %f %f %f ');
  if (debug > 0)
    fprintf(1, 'num_lines %3d  num_read %4d \n', num_lines, num_read);
  end
  if (num_read ~= 4) 
    break;
  end

  % okay, that line contained valid data.  Store in arrays
  num_lines = num_lines + 1;
  x_array(num_lines) = data_array(2);
  y_array(num_lines) = data_array(4);
end

% now, make the plot
plot(x_array, y_array, 'o');
title('Simple graph from data in a file');